Actuators of the type which include a piston movable in a housing are used to position or move various loads, such as the control surfaces of an aircraft. For reasons of safety, in aircraft applications dual actuators are employed to control the position of a control surface. If one actuator fails, the other actuator positions the control surface. However, because the pistons of the two actuators are movable together, if the movement of one piston is restricted or prevented, neither actuator can function properly.
In a typical actuator of this type, a connecting rod is joined to the piston and extends through an opening in a tubular member of gland. The motion of the piston is transmitted by way of the connecting rod to an output member which may be an aircraft control surface. One kind of damage which can lead to malfunctioning of both actuators is damage to the connecting rod of the type which forms an abutment or projection on the connecting rod. For example, in military aircraft, this may be caused by an enemy projectile passing through the connecting rod with consequent deformation of the rod to form the projection. In this event, movement of the piston will cause the projection on the rod to engage the tubular member. This engagement prevents further movement of the piston and rod in that direction and may cause jamming of the actuator. In either event, the advantages of redundant actuators are lost.
Simmons U.S. Pat. No. 3,884,127 solves this problem by providing a frangible inner periphery on the tubular member. With this construction, when the projection on the rod strikes the frangible inner periphery of the tubular member, the frangible inner periphery breaks to permit the rod with the projection thereon to pass through the opening in the tubular member. Consequently, even though the damaged actuator is no longer able to control the position of the control surface, it does not prevent the undamaged actuator from controlling the position of the control surface.
Although the patented construction is very satisfactory, the frangible inner periphery is not alone suitable to serve as a bearing for the rod. To incorporate bearing features, a three-piece gland which has three separate frangible sections has been used. When the projection is formed on the rod axially inwardly of the gland, then outward movement of the rod sequentially breaks two of the frangible sections to permit movement of one piece of the gland axially outwardly with the rod. Conversely, when the projection is formed on the rod axially outwardly of the gland, axial inward movement of the rod causes the fracture of two of the frangible sections and the inward movement of two of the pieces of the gland.
While this construction is desirable because it gives the gland the desired bearing capacity, it is relatively costly and complex. In addition, it increases the weight penalty on the aircraft. Finally, it has been found that certain projectiles which strike the housing of the actuator may have insufficient force to pass completely through the actuator. These projectiles may become lodged in both the actuator housing and the rod. Such projectiles have the tendency to "nail" the rod to the housing with the result that the frangible sections never have an opportunity to perform their function.